The present invention relates to artificial heart valves, and more particularly to flexible leaflet heart valves which are used to replace the natural aortic or pulmonary valves of the heart.
Conventionally, ball or disk valves are used to replace natural mitral or tricuspid aortic or pulmonary valves of the heart. These artificial valves comprise a rigid frame defining an aperture and a cage enclosing a ball or a disk. When blood flows in the desired direction, the ball or disk lifts away from the frame allowing the blood to flow through the aperture. The ball or disk is restrained by the cage by struts or by a pivot. When blood tries to flow in the reverse direction, the ball or disk becomes seated over the aperture and prevents the flow of blood through the valve. The disadvantage of these valves is that the ball or disk remains in the blood stream when the blood flows in the desired direction, and this causes a disturbance to blood flow.
More recently, flexible leaflet valves have been proposed which mirror natural heart valves more closely. These valves have a generally rigid frame and flexible leaflets attached to this frame. The leaflets are arranged so that, in the closed position, each leaflet contacts its neighbour thereby closing the valve and preventing the flow of blood. In the open position, the leaflets separate from each other, and radially open out towards the inner walls of an artery in which the valve is located. The leaflets are either made from chemically treated animal tissue or polyurethane material. The leaflets must be capable of withstanding a high back pressure across the valve when they are in the closed position, yet must be capable of opening with the minimum pressure across the valve in the forward direction. This is necessary to ensure that the valve continues to correctly operate even when the blood flow is low, and to ensure that the valve opens quickly when blood flows in the desired direction.
A wide range of geometries are used to describe natural aortic valve leaflets during diastole, but these geometries cannot be used for valves made from pericardial or synthetic materials due to the approximately isotropic properties of such materials compared to the highly anistropic material of the natural valve. Consequently, different geometries have to be used to form flexible leaflet heart valves made from pericardial or synthetic materials with isotropic mechanical properties.
Conventional flexible leaflet heart valves have three substantially identical leaflets mounted onto the frame. The leaflets have a range of designs, both in the geometry of the leaflet and the variations in thickness of the leaflets. Original flexible leaflet heart valves incorporate leaflets which are spherical or conical when in the relaxed state, that is when no pressure is acting on the leaflet. More recently, cylindrical and ellipsoidal leaflets have been proposed. These leaflet geometries are formed with an axis of revolution in a plane generally parallel to the blood flow through the valve.